Operation OAU begins during the Nigerian Civil War.

Operation OAU was a battle between Nigerian 3 Marine Commando Division (3MCDO) and Biafran 12 Division in modern day south-eastern Nigeria. Operation OAU was an intermittent battle that may have resulted in over 25,000 deaths on both sides. Although the Biafran soldiers were outnumbered, they were able to retain control of Umuahia and eventually recapture the cities of Owerri and Aba.

The Nigerian Civil War, also known as the Biafran War, Nigeria-Biafra War, or Biafra War, was fought between Nigeria and the Republic of Biafra, a secessionist state which had declared its independence from Nigeria in 1967. Nigeria was led by General Yakubu Gowon, and Biafra by Lieutenant Colonel Chukwuemeka "Emeka" Odumegwu Ojukwu. The conflict resulted from political, ethnic, cultural and religious tensions which preceded the United Kingdom's formal decolonisation of Nigeria from 1960 to 1963. Immediate causes of the war in 1966 included a military coup, a counter-coup, and anti-Igbo pogroms in the Northern Region. The pogroms and the exodus of surviving Igbos from the Northern Region to the Igbo homelands in the Eastern Region led the leadership of the Eastern Region to conclude that the Nigerian federal government would not protect them and that they must protect themselves in an independent Biafra.